The alveolar epithelium is comprised of 2 morphologically distinct differentiated epithelial cells, type I (TI) and type II (TII) cells, both of which are thought to be critical for normal lung function. Although the establishment, maintenance, and repair of a normal alveolar epithelium are essential for mammalian life, we have a limited understanding of the important processes that regulate these events. The broad objectives of the research are to understand how alveolar epithelial maintenance and repair are regulated. Within this context, the overall goals of this project are to elucidate the fates of TI and Tll cells and the cellular mechanisms responsible for the transdifferentiation of both cell types following lung injury. Studies performed 30 years ago using techniques of autoradiography and electron microscopy generated the current paradigm that after lung injury TII cells proliferate and transdifferentiate into TI cells. Based on studies in the literature and work presented in the Progress Report and Preliminary Data, the current paradigm appears now to be incomplete. There are reports in the published literature that TI cells may have reparative potential, and there has been recent speculation that epithelial-mesenchymal transformation may occur in the lung, in a similar fashion to the pathogenesis of renal fibrosis. Immunohistochemical data presented in this application provide additional evidence that transdifferentiation occurs between the mesenchymal and epithelial compartments. Recently, we found that TI cells proliferate in vitro, supporting the concept that TI cells may also participate in the repair process in vivo. These observations have prompted us to generate the working hypothesis that repair following lung injury is more complex than a simple linear progression from mature TII to TI cells, involving other cellular processes such as transdifferentiation among TI cells, TII cells, and mesenchymal cells, and the proliferation of TI cells. The proposed studies comprise 3 specific aims to determine the extent to which TI and TII cells exhibit plasticity of expression and to elucidate the mechanisms for transdifferentiation. 1) To determine the fates of TI and TII cells following injury by fate-mapping studies in transgenic mice; 2) To ascertain at various time points in lung injury the molecular phenotypes and proliferative properties of various populations of alveolar epithelial cells, including "intermediate cells," transitional cells expressing markers of both TI and TII cell phenotypes; and 3) To evaluate candidate regulatory genes by Q-PCR, by in situ hybridization, and by functional studies in vitro and in vivo.